1. Field of the Invention
The present invention relates to a side light type spread illuminating apparatus, and more particularly to a spread illuminating apparatus used as a lighting means for a liquid crystal display device.
2. Description of the Related Art
A side light type spread illuminating apparatus, in which a primary light source is disposed at a side surface of a light conductor plate, is predominantly used as a lighting means for a liquid crystal display (LCD) device used in a mobile telephone, and like devices. Conventionally, the primary light source has been constituted by a cold cathode lamp. Currently, a point light source, such as a white light emitting diode (LED), which is easier to handle, enables easier downsizing, and is more resistant to impact shock than the cold cathode lamp, is heavily used. The application of a spread illuminating apparatus using such a point light source is expanding beyond usage in a small LCD device for a mobile telephone, and is now considered for usage in a relatively large LCD device for a car navigation system.
In order to satisfactorily cover an increased illumination area in an enlarged LCD device, it is desirable to apply an increased current to the point light source thereby increasing the amount of light emitted from the point light source. The increased current applied to the point light source, however, causes an increase of heat thus raising temperature, which lowers the luminous efficiency of the point light source.
To overcome such a problem, various methods are considered to efficiently allow heat generated by the point light source to escape outside. For example, a spread illuminating apparatus 1 shown in FIG. 5 includes a light conductor plate 2, LEDs 3 as point light sources mounted on a flexible printed circuit board (hereinafter, referred to as FPC as appropriate) 4 and disposed at a side surface (light inlet surface) 2a of the light conductor plate 2, and a frame 5 adapted to hold together the components described above and made of a metallic material having an excellent heat conductance, e.g., aluminum. In the spread illuminating apparatus 1 as described above, in order to more efficiently release heat outside, a mechanism for radiating heats from the LEDs 3 is disclosed which is structured such that holes are formed through both the FPC 4 and a heat radiating plate 5a made of metal and disposed behind the FPC 4, and adhesive filling material of high heat conductivity is put in the holes (refer to, for example, Japanese Patent Application Laid-Open No. 2002-162626). In such a heat radiating mechanism, the bottom face of a point light source, which is a package surface located closest to a light emitting portion to most generate heat, is connected to a metallic frame to support the point light source by means of adhesive filling material having a higher heat conductance than an FPC, whereby heat radiation characteristic is enhanced.
In the heat radiating mechanism described above, the heat generated by the point light source mounted on the FPC is conducted generally through electrode terminals of the point light sources, electrical conductors of the FPC, and a base material (for example, a base film) of the FPC before getting to the rear surface of the FPC and then to the heat radiating plate connected to the rear surface of the FPC, thus failing to provide an adequate heat radiating pathway to efficiently radiate the heat emitted from the point light source and also failing to provide a sufficient area for direct radiation. Especially, in case of a large current LED, a large amount of heat is emitted from its package, thus aggravating the heat problem.
Also, in a conventional spread illuminating apparatus, it may possibly happen that a point light source mounted on an FPC is lifted off due to solder applied to a land. For achieving a favorable coupling efficiency between a light conductor plate and a point light source, it is required that the point light source be precisely positioned relative to the light conductor plate with respect to two directions oriented orthogonal respectively to a major surface and a light inlet surface of the light conductor plate. If the lift-off problem occurs in the spread illuminating apparatus 1 shown in FIG. 5, variation is caused in the height position of the LEDs 3 from the FPC 4, in which case the coupling efficiency is decreased regardless of the FPC 4 arranged parallel to the light inlet surface 2a of the light conductor plate 2 (as shown in FIG. 5) or arranged orthogonal thereto. Under the circumstances described above, a point light source is becoming smaller and smaller in dimension thus making its effective mounting area smaller, and therefore it is becoming important to increase the mounting strength of a point light source for the purpose of securing a stable optical coupling efficiency over a long period of time. Consequently, it is requested to resolve deficiency in strength for mounting a point light source.